1. Technical Field of the Invention
This invention relates to a shielded strip line device to be mounted on a circuit board or an electronic substrate and a method of manufacture thereof, and particularly relates to a shielded strip line device, which is favorably adapted to high speeds and high frequencies, mainly for use as a noise filter bypass device or power decoupling device, and a method of manufacturing such a shielded strip line device.
2. Description of the Related Art
With the progress of scientific technologies, the making of electronic equipment more compact and higher in performance is being demanded. This is achieved, for example in switching power supplies and digital signal processing circuit parts, by making the clock frequency higher. However, this causes an increase in the high frequency current that flows through the circuit, in particular the power supply circuit, causing significant increase in electromagnetic radiation and lowering of signal quality and thus placing more difficult demands on the performance of noise filter bypass devices and power decoupling devices.
As power decoupling devices for high-speed digital circuits, ceramic capacitors, formed by laminating a plurality of layers of ceramic material having a thin metal film deposited thereon, and solid-state electrolytic capacitors, having a porous formed body of a valve metal, such as tantalum or aluminum, etc., as an anode, the oxide film of this porous formed body as a dielectric, and a conductive polymer as a solid-state electrolyte, have been developed priorly.
As an example of a solid-state electrolytic capacitor, Japanese Patent Publication No. Hei-4-56445 (Japanese Unexamined Patent Publication No. Sho-60-37114) discloses a solid-state electrolytic capacitor having polypyrrole or an alkyl-substituted form thereof disposed as a solid-state electrolyte on a dielectric oxide film. Also, Japanese Unexamined Patent Publication No. Hei-3-35516 discloses a solid-state electrolytic capacitor, with which polyaniline is formed as a solid-state electrolyte on a dielectric oxide film, and a method of manufacturing such a solid-state electrolytic capacitor. With each of these capacitors, a conductive polymer that is 100 times or more higher in conductivity in comparison to prior capacitors is used as the solid-state electrolyte. These capacitors are thus small in equivalent serial resistance and, in comparison to prior capacitors of the same capacitance, exhibit effects up to a high frequency range that is 100 times or more higher in frequency. However, even these capacitors increase dramatically in impedance and cannot meet recent demands as filter bypass devices and power decoupling devices in high frequency ranges in the excess of 10 MHz.
Meanwhile, filter arrangements for accommodating high frequencies are also being examined. Japanese Unexamined Patent Publication No. Hei-6-53046 discloses a surface-mounted noise filter, comprising a meandering conductor and a ground conductor, each of which is sandwiched by ceramic dielectric sheets. FIG. 1 is a sectional view, which shows the arrangement of this prior-art surface-mounted filter. As shown in FIG. 1, with this prior-art surface-mounted filter, a first dielectric sheet 110, second dielectric sheet 120, and third dielectric sheet 130, which are, respectively, rectangular form, are laminated to form a laminated body 153. A first signal electrode 151 and a second signal electrode 152 are attached respectively to each of the pair of mutually opposing end faces among the end faces parallel to the direction of lamination of laminated body 153.
A first internal conductor 111, second internal conductor 112, and meandering conductor 115, which are used for signal transmission, are disposed between first dielectric sheet 110 and second dielectric sheet 120. First internal conductor 111 is connected to first signal electrode 151, second internal conductor 112 is connected to second signal electrode 152, and meandering conductor 115 is connected between first internal conductor 111 and second internal conductor 112. A ground conductor 125, which opposes meandering conductor 115, is disposed between second conductive sheet 120 and third conductive sheet 130, and ground conductor 125 is connected to a pair of ground electrodes (not shown). These ground electrodes are attached to a pair of end faces, among the end faces of laminated body 153 that are parallel to the direction of lamination of laminated body 153, to which first signal electrode 151 and second signal electrode 152 are not attached. An inductance is formed at meandering conductor 115 and a capacitance is formed across meandering conductor 115 and ground conductor 125. A noise filter, which combines an inductance device and a capacitance device, is thus formed and noise filter that is excellent in noise absorption characteristics at high frequencies is thus obtained. With this surface-mounted filter, an electric signal that is input from first signal electrode 151 is filtered by passage through first internal conductor 111, meandering conductor 115, and second internal conductor 112 and output from second signal electrode 152.
However, this prior art has the following problem. Though capacitors, having an abovementioned conductive polymer as a solid-state electrolyte, are used in various applications as capacitors that can be used up to a high-frequency range, even with such capacitors, the impedance increases drastically in a high-frequency range in the excess of 10 MHz. Thus under operation at a clock frequency of several hundred MHz, which is generally implemented in digital circuits, the characteristic assumed for a signal generating circuit, that is, the characteristic that the power impedance is infinitely close to zero regardless of frequency cannot be realized as long as such a capacitor is used. As a result, these capacitors cannot meet recent demands as filter bypass devices or power decoupling devices. Also, though surface-mounted filters for the purpose of noise elimination have also been developed, these cannot realize an infinitely low impedance value and are thus limited in use as substitutes for capacitors. It is thus difficult to realize low impedance especially in high-frequency regions of 100 MHz or more with such filters.